I had mentioned in New AOM & The Sandbox some of the pre-existing add-ons (such as Resizeable Textarea) had been in placed in the sandbox. This was causing users to get an “Add-on not found!” message when they tried to follow a link to an existing add-on that was in the sandbox. Based on user feedback this has changed:

All add-ons that were public in AMO prior to Remora’s launch this weekend should now be publicly available on AMO again.

News Source: Mozilla Webdev